Let's say you were a native-born citizen, born to an average family (neiter poor nor well-off).

Please specify why (ie, if you're a woman, is it for more rights in this society, as opposed to others), and also which period of time you'd like to live in (ie, Athens during the Golden Age, Rome during the Principate, etc).

Being a middle-class citizen in the Maya civilization sounds like a pretty good deal. None of the risk and downsides to being a noble, none of the labor and drudgery of being a peasant farmer. I could easily make a living as a trader, which would give me the possibility of being comfortably wealthy and respected and it would allow me to see more of the world than most people comparatively speaking. Best of all I get to eat Mexican food every meal of every day!

For sure a very difficult decision. I pick Sumer because I think I would enjoy the innocence of worshiping gods and goddesses, bathing and dressing statues of them and taking them to visit gods and goddesses in other villages. To me this sounds pretty much like Eden before Eve ate the forbidden fruit.

Gaelic Civilisation if it's 7th Century or later, Rome for the period 100 BC - 200 AD, and perhaps one of the Greek cities if I had to go earlier. Each of these have their peculiarities, and to be honest I'm not sure I'd like to live in any of them really.